The lung grows by adding new alveoli during early development and by enlarging them later on, yet little is known about what causes the switch from alveolar proliferation to hypertrophy. Although compensatory pulmonary growth following unilateral pneumonectomy may be achieved by the formation of extra alveoli in immature mammals, alveolar hypertrophy is the rule in mature ones. To explore these two modes of development, the kinetics of cellular and alveolar hyperplasia and hypertrophy will be measured during normal ontogeny and compensatory growth of lungs in inmature and adult rats by autoradiographic and histometric methods. In order to explore the physiological stimuli to these adaptive changes in pulmonary architecture, lungs will be unilaterally incapacitated by bronchial occlusion, pulmonary arterial ligation or atelectasis to compare the responses of the opposite lungs with their degree of compensation following pneumonectomy. Coupled with exposure to high or low oxygen tensions, these experiments are designed to reveal how the size of the lung is regulated by functional demands and also how alterations in pulmonary physiology affect the right side of the heart. In the same vein, the possible enlargement of fetal lungs in response to maternal pneumonectomy--or to the removal of one lung in utero--will be investigated for clues to the control of lung growth during prefunctional stages of development.